Competing Ions in Electrolysis (Oxford AQA IGCSE Chemistry): Revision Note

Competing Ions in Electrolysis

• During the electrolysis of aqueous electrolytes there will be water present

• Some water molecules split up into hydrogen and hydroxide ions, H⁺ and OH^–, which participate in the electrolysis reactions

H₂O (l) H⁺ (aq) + OH^– (aq)

Rules for identifying products formed at electrodes

Positive Electrode - Anode

• Negatively charged OH^– ions and non-metal ions are attracted to the positive electrode

• If halide ions (Cl^-, Br^-, I^-) and OH^- are present then the halide ion is discharged at the anode, loses electrons and forms a halogen (chlorine, bromine or iodine)

• If no halide ions are present, then OH^- is discharged at the anode, loses electrons and forms oxygen gas

• In both cases, the other negative ion remains in solution

Negative Electrode - Cathode

• H⁺ ions and metal ions are attracted to the negative electrode but only one will gain electrons

• Either hydrogen or a metal will be produced

• If the metal is above hydrogen in reactivity series, hydrogen will be produced – bubbling will be seen at the cathode

The reactivity series

Using Named Electrolytes

• You must be able to identify the products at the electrodes from solutions of copper chloride and sodium chloride

The Products of Electrolysing Aqueous Solutions

Explaining the products:

• Copper chloride:

  • Copper is below hydrogen in the reactivity series so copper(II) ions are preferentially discharged at the cathode; chlorine is a halogen, so is preferentially discharged at the anode

• Sodium chloride:

  • Sodium is above hydrogen in the reactivity series so hydrogen ions are preferentially discharged at the cathode; chlorine is a halogen, so is preferentially discharged at the anode